Separation of xylene isomers by liquid thermal diffusion



Sept. 16, 1958 Filed April 12, 1954 REFRACTIVE INDEX (AT 25C) I.493 I |I I I I i x I I -:.MTA-'?.'3A..J!- I I I I /ORTHO-META XYLENE L495 I I I I I L496 I I RTHO-PARA XYLENE l I I I I I I L499 L 0 I 2 a 4-5 a 1'9 9 IO SEPARATION OF XYLENE ISOMERS BY LIQUID R A. GARDNER ETAL 2,852,578

THERMAL DIFFUSION 5 Sheets-Sheet 1 FIG.I

SEPARATION OF BINARY XYLENE ISOMER SYSTEMS BY LIQUID THERMAL DIFFUSION INVENTORS RALPH A. GARDNER & y CHARLES W.SEELBAOH POSITION IN COLUMN BOTTOM Sept. 16, 1958 Filed April 12, 1954 R. A. GARDNER ETAL 2,852,578

SEPARATION OF XYLENE ISOMERS BY LIQUID THERMAL DIFFUSION 3 Sheets-Sheet 2 FIG. 2

SEPARATION OF TERNARY XYLENE ISOMER MIXTURE BY LIQUID THERMAL DIFFUSION 19s HOURS CH CH CH H 3 /PARA-'FIN IMPURITY l I 2 3 .4 s s 1 a 9 IO TOP POSITION m COLUMN BOTTOM INVENTORS RALPH A.GARDNER 8: BY CHARLES W. SEELBACH ATT RNEY Sept. 16, 1958 R. A. GARDNER ETAL 2,852,578

- SEPARATION OF XYLENE ISOMERS BY LIQUID THERMAL DIFFUSION Filed April 12, 1954 3 Sheets-Sheet 3 FIG.3.

VOLUME RATIO OF PARA XYLENE TO META XYLENE AFTER SEPARATION BY LIQUID THERMAL DIFFUSION IN PRESENCE OF ORTHO XYLENE -TERNARY MIXTURE LOO PARA/ META-- VOLUME RATIO BOTTOM POSITION IN COLUMN INVENTORS RALPH A. GARDNER a y CHARLES W. SEELBACH AT ORNEY United States Patent '0 SEPARATION OF 'XYLENE'ISOMERSBY LIQUID THERMAL DIFFUSION.

This invention relates to a process for the separation of xylene isomers by liquid thermal diffusion and more particularly to the separation of mixtures of meta xylene and para xylene in the presence of ortho xylene by liquid thermal diffusion.

There is a growing demand for the pure isomers of xylene. Para xylene is a row material used in the production of terephthalic acid which, in turn, is used in the production of a well-known synthetic fiber. being made to use o-xylene in the production .of phthalic anhydride and to use m-xylene'in the production of meta phthalic acid.

Unfortunately the methods'of producing crude xylene in commercial quantities yield mixtures .ofits isomers. For example, the reforming of a 155-406 F. cut from a Mid-Continent crude oil over a chromia-altunina bead catalyst may yield a normally liquid product containing 6.6% xylene which is composed of 24% o-xylene, 21% p-xylene, and 55% m-xylene.

It is not too difficult to separate o-xylene from this xylene mixture because of the 9.5 F. spread between the boiling point of o-xylene and the nearest of the other two isomers. The big problem is to separate the meta and para isomers whose respective boiling points differ by only 1.4 F. This spread is too small to justify commercially their further separation'by fractional distillation. Fractional crystallization appears to be a logical choice because their melting points differ by 110 F. However, an eutectic rnixture is formed at about 63 F. when the concentration of p-xylene is about 13% by weight. Thus, when using fractional crystallization, a portion of p-xylene cannot be recovered. This loss can 'be reduced-by increasing the concentration of p-xylene in discovered that theseisomers are .readilyseparableby liquid thermal diffusion whenin 'the-presence of ortho xylene. The ternary system of xylene'isomers maycontain as impurities other hydrocarbons without affecting the invention. 1 x

The relative proportion or ratio of p-xylene to m-xylene may vary widely, but for the .best separations the ratio should fall on a volume .basisinihe range of .0..3 to 3.0. It is also possible to improve the degree of'separation of the para and meta isomers by the presenceofany amount of ortho xylene. However, for best separations the con-. centration of ortho xylene should range from to 90% based on the total volumn of liquid to be treated, and preferably from to 50%.

Neither the type of liquid thermal diffusion apparatus nor the flow pattern used therein are critical to this invention. The continuous flow apparatus disclosed in U. S. Patent No. 2,541,069 or the static columns .of copending U. S. application S. N. 278,848. filed Marchl,

1952, now Patent No. 2,742,154, may be used.

Efforts are Patented Sept. 16, 1958 conditions in obtaining all the data-presented below. It is similar to that shown in Figure 1 of .copending U. S. application S. N. 278,848. Its annular "slit, formed by two concentric tubes, has an effective height of five feet, a width of 0.0115", and a volumetric capacity-of 22.2 cc. Ten withdrawal ports are spaced six inchesapart, beginning at the bottom. The slit walls were maintained at 115 and 70 F., respectively. At the .end of agiven period of time, ten liquid fractions were removed by'first opening the top withdrawal port and draining the.column to that level, and then successively following the same procedure for each withdrawal port in descending order of height until ten fractions were obtained.

The above static apparatus is extremely useful for separating small amounts of materialsby thermal diffusion and to assay the potentialities ofthe process in separating components of any particular material. If a material separates in a static column, it can also be separated continuously ina thermal diffusion apparatus, and this factis the reason why the type of thermal diffusion apparatus is not critical. The rate and degree of separation inall types of such apparatuses are influenced by many factors which are likewise not critical to this invennon.

It is well known that the composition of any liquid is related to its refractive index. A change in composition can be measured and expressed in terms of a change in refractive index. Therefore, except where-quantitative results arerequired, compositions were measured and are so reported in the examples in terms of refractive index because of the speed and easeof makingsuch measure ments.

Ourinvention will .be better understood from the following examples and drawings in which Figure 1 is a'plot in ditferentpositionsof a thermal diffusion column of refractive index or .composition of binary xylene isomer systems after 48 hours in a liquid thermal diffusion column;

Figure 2 is a plot in different positions of a thermal diffusion column of xylene isomer concentrations in a ternary xylene isomer system after 196 hoursin the column; and

Figure 3 is a graph of parato'meta xylene isomer ratios based on the data shown in Figures 1 and 2.

EXAMPLES l-3 Thethree possible binary xylene mixtures, ortho-para, ortho-meta, and meta-para were prepared from isomers currently soldas chemically pure. The comparison in Table I of their actual refractive indices at 25 1 C. With literature values indicates the commercially 'pure isomers had a slight amount of impurities.

Each binary mixture comprising equal parts'by volume of two different isomers was charged to the column. At the end of 48 hours, ten liquid fractions were withdrawn and their refractive indices -measured. "Results are tabulated in Tablel.

3 Table I These results, plotted in Figure 1, show that the orthopara and ortho-meta pairs are separable by liquid thermal difiusion, but that the meta-para mixture is not separable to any practical extent. For example, expressing percentage separation as the value obtained by dividing the refractive index difference between. the top and bottom fractions by the same diiference between the chemically pure isomers used in blending the binary mixtures, we find that the ortho-para mixture underwent a or 92% separation. The separation value for the orthometa mixture is 113%. Separations in excess of 100% are explained by the presence of impurities, as can be seen by the fact that the refractive index of the top fraction of the ortho-meta mixture is (1.49461.4931) or 0.0015 numbers below that theoretically possible. The separation value for the meta-para mixture is only 13 a value so low, aside from any errors due to impurities, as to indicate that these cannot be separated by thermal diffusion on an economic basis.

Actual compositions of the top and bottom fractions in Examples 1-3 were determined by infra red analyses. Results are set forth in Table B below.

Table B Fraction (m'p) Volume Percent EXAMPLES 4 AND 5 1 Ternary mixtures comprising equal volumes of each of the three chemically pure isomers were processed in the same apparatus and under the same conditions as in Examples 13. Fractions were withdrawn after 48 hours in the first instance, and after 196 hours in the second. Results are shown in Table C below.

Table C Example 4 Example 5 Fraction (48 hrs.) (196 hrs.)

0 1. 4982 1. 4980 10 (bottom 10 percent) 1. 5000 1. 4983 These figures in Table C are significant only in that they show something has been separated. To determine what has been separated, infra red analyses of the ten fractions obtained in Example 5 were made and the results are set forth in Table II and plotted in Figure 2.

Table II Volume Percent Fraction Ortho Meta Para Total 1 (top 10%) 6. 3 27. 7 54. 5 1 88. 5 2 10. 2 33. 4 51. 4 1 95. 0 16. 0 38. 0 50. 3 104. 3 23. 8 39. 2 41. 2 104. 2 27.2 38.4 35.4 101.0 31. 9 38. 9 31. 6 102. 4 33. 4 38. 3 28. 3 100.0 39. 3 37. 6 22. 7 99. 5 2 53. 2 32. 5 14. 3 100.0 2 75.7 19. 2 5.1 100.0

1 Low because of impurity. 2 Determined by difierence.

Table II shows that the upper part of the column has become enriched in para xylene and the bottom of meta xylene.

The unexpected separation of the meta and para isomers of xylene in the presence of ortho xylene by liquid thermal difiusion is shown more effectively by comparing the volume ratios of para to meta in each fraction of Examples 3 and 5. These ratios are set forth in Table III and plotted in Figure 3.

Table III Volume ratio; Para] Meta Fraction Example 3 Example 5 Binary Ternary Mixture Mixture tion. The invention, therefore, includes the treatment of a crude xylene mixture by liquid thermal diifusion alone or in combination with other methods of separation.

We claim:

1. A process for separating a solution of para xylene and meta xylene in the liquid phase which comprises subjecting said solution to thermal diffusion in the presence of to 90% of ortho xylene based on the volume of the solution, whereby at least one fraction is enriched in para xylene and at least a second fraction is enriched in meta xylene.

2. The process of claim 1 in which the top fraction in a vertical liquid thermal diffusion process is enriched in para xylene.

3. The process of claim 1 in which the para xylene enriched fraction is subjected to a process of fractional crystallization.

4. The process of claim 1 in which the para xylene enriched fraction is further subjected to a process of fractional distillation, whereby the ortho xylene concentration of the overhead fraction is reduced.

5. The process of claim 4 in which the overhead frac- 6 tion is subjected to a process of fractional crystallization.

6. A process for separating para xylene from a mix ture thereof with meta xylene in a volume ratio of 0.3 to 3.0, which comprises subjecting said mixture in the presence of to volume percent ortho xylene based on the total volume of the solution, in the liquid phase, to thermal difiusion to separate the mixture into two fractions, one of which is enriched in meta xylene and the other of which is enriched in para xylene.

References Cited in the file of this patent UNITED STATES PATENTS 2,511,711 Hetzner et al June 13, 1950 2,541,069 Jones et al Feb. 13, 1951 2,541,071 Jones et al. Feb. 13, 1951 OTHER REFERENCES Jones et 211.: Ind. & Eng. Chem., vol. 45, pp. 2689- 2696 (December 1953). 

1. A PROCESS FOR SEPARATING A SOLUTION OF PARA XYLENE AND META XYLENE IN THE LIQUID PHASE WHICH COMPRISES SUBJECTING SAID SOLUTION TO THERMAL DIFFUSION IN THE PRESENCE OF 10 TO 90% OF ORTHO XYLENE BASED ON THE VOLUME OF THE SOLUTION, WHEREBY AT LEAST ONE FRACTION IS ENRICHED IN PARA XYLENE AND AT LEAST A SECOND FRACTION IS ENRICHED IN META XYLENE. 